1. Field of the Invention
The present invention relates to the field of still video imaging, and especially to a focusing arrangement for a still video camera of the type that produces a movie image for an electronic viewfinder in addition to a still image for recording.
2. Description Relative to the Prior Art
A still video camera, by virtue of its inherent video processing capability, is well suited for combination with an electronic viewfinder (see, for example, the camera disclosed in U.S. Pat. No. 4,456,931). This combination is particularly advantageous for previewing the picture as it will actually appear subsequent to video processing. Such a camera operates in two modes: a movie mode for producing a moving video image in the viewfinder and a still mode for producing and recording a still image. The movie mode is comparable to the operation of any motion picture video camera. In such a camera, the photosensitive region of an image sensor is continuously irradiated by scene light. Since the exposure time is fixed to accord with the video image rate (e.g., 1/30th second), the amount of light striking the image sensor is controlled by regulating the optical aperture presented to the incoming light with an adjustable diaphragm.
In the still mode, it is desirable that both the exposure time and the optical aperture are controlled so as to vary, respectively, the length of exposure and the intensity of the light striking the image sensor. For example, in order to obtain a distinct still image of an object in motion, it is necessary to shorten the exposure time so as to prevent image blur. In shortening the exposure time it is ordinarily necessary to open the aperture so as to maintain the same total exposure. On the other hand, in order to obtain a distinct still image over a broad distance range, it is necessary to close down the aperture so as to extend the depth of field. Then the exposure time would have to be lengthened accordingly. The adjustments are thus interactive, that is, exposure time adjustments require corresponding aperture adjustments, and vice-versa, in order to maintain a correct exposure value.
In a manually focused video camera, an electronic viewfinder is also used to determine when the image is properly focused. The proper focus, however, is not a discrete distance but a range of distances through which the image is acceptably defined, i.e., a range determined by the depth of field for a particular focal length, optical aperture, and subject distance. An electronic viewfinder repeats images of the scene at the television frame rate, i.e., 1/30 second. This "exposure interval" dictates a particular lens aperture for the existing light condition and, therefore, establishes a particular depth of field for each subject distance. A special problem emerges when the viewfinder is used with a video still camera because the light condition that dictates a certain aperture . . . and thus a certain depth of field . . . for the electronic viewfinder may require an entirely different aperture for the still exposure (because the exposure interval may not be 1/30 second). Thus the depth of field for viewing may be an artificial indication of the actual picture-taking condition. The "real" depth of field may be considerably less than that observed in the viewfinder with the attendant possibility that a properly-focused subject in the viewfinder may be unfocused (that is, outside the range of focus) with respect to the recorded picture.
If one could reliably locate the central focus position for the displayed depth of field, the picture would be in focus for any aperture (that is, for any depth of field). Owing, however, to the small size of the display screen in the viewfinder and the limited bandwidth available for the display, differences between details shown on the viewfinder screen are not very sharp or clear. Searching for optimum focus, necessary in view of the depth of field considerations heretofore mentioned, is a procedure marked by uncertainty. Perhaps the most elegant solution is to use a ranging device, such as the ultrasonic generator suggested in U.S. Pat No. 4,463,384, and then to couple the distance measurement to an indicator in the viewfinder. A costly and complex ranging system, however, is undesirable if cost and simplicity are significant factors.
U.S. Pat. No. 4,481,540 suggests another arrangement for dealing with such focus problems, but without dependence upon a distance measuring device. Two versions of the video signal, one unmodified and the other blurred by a low pass filter, are applied to the viewfinder via a switching device controlled by the high frequency content of the video signal. The unmodified part of the image is switched to the display during the presence of high frequency content while the blurred part of the image is displayed otherwise. As the image is brought into focus, more unmodified image, and less of the intentionally blurred image, appears in the viewfinder.
The focusing arrangement in the U.S. Pat. No. 4,481,546 however, requires a special viewing mode that occupies the image field and otherwise detracts from the main purpose of the viewfinder . . . to observe the scene as recorded by the camera. Moreover, the relatively low bandwidth of the electronic viewfinder ordinarily limits the rendition of high frequency, well-focused picture information. The visually-observed difference between the blurred and the unmodified but focused image will often be insignificant, particularly in poor lighting. Furthermore, the switching between the blurred and unmodified image requires not only a low pass filter (for blurring) and an electronic switch, but also requires a focus-sensitive trigger for the switch. (In the case of the U.S. Pat. No. 4,481,540 the trigger signal is derived from an aperture correction circuit.) All of this again adds complexity and cost to the camera.